Fluctuates relative to what? If BTC becomes the unit of account (as it already apparently has in certain channels of international trade), then it’s only fiat money doing the fluctuating, while goods and services are priced in BTC.
In other words: the mass public is unlikely to become Bitcoin-first until they can gain confidence in the stability of Bitcoin. This is notwithstanding the fact that stable governments likely will not accept BTC as a national currency, meaning citizens will still need USD/EUR/GBP/their local currency for paying taxes and transacting with the government.
It feels as though your post just handwaved away all of the practical issues with actually making the world BTC-first and then asked why I cared about them. Because, in order for the world you're proposing to exist, we almost certainly have had to solve them.
Classic example of buying a good using BTC: say it costs 100 satoshis. By the time I click and purchase, and by the time the BTC arrives at the seller's address, the value may have changed for the better (BTC price goes down - hence I paid "less") or for the worse (BTC price goes up - hence I paid "more"). For the most extreme example, imagine how people that bought a pizza with Bitcoin when it was worth 5$ feel right now. A 15$ pizza in 2009 or whatever is worth ~150K USD now.
The pizza was 10k BTC (now ~$443M). Tesla bought about 25k BTC.
A 15$ pizza in 2009 or whatever is worth 18.10$ now.
So which currency should you be spending or which one should you be saving?
When the amount of goods and services you can buy in any country swings by 10% in a day, you’re nowhere near being treated like a currency.